This invention relates to the inhibition of combustion of some surfaces of solid propellant charges for or in rocket motors, and is concerned with methods of providing such inhibition and solid propellant charges which are so inhibited.
Rocket motors can be provided with charges of solid propellant in either of two ways. The first of these is by the preparation, externally of the case of the motor, of a solid propellant grain which is subsequently inserted in the combustion chamber in the case, whereby the motor is said to be "cartridge loaded". The second of these is by the casting of the propellant within the combustion chamber, the wall of the combustion chamber which is usually the case of the motor, acting as the whole or part of the mould, and the propellant bonding to the wall, whereby the motor is said to be "case bonded".
For cartridge loaded rocket motors, virtually all of the surfaces of the solid propellant grain are exposed to the environment of the rocket combustion chamber, so that it is usually desirable to inhibit combustion at some of the surfaces. Often inhibition is also required for some of the otherwise exposed propellant surfaces of a case bonded rocket motor.
Cellulose derivatives, typically esters and ethers such as cellulose acetate and ethyl cellulose, have been used for many years for inhibiting combustion. However, they suffer from the disadvantage that, when used for inhibiting the combustion of solid propellants containing nitro glycerine or other nitric esters and/or plasticizers, they absorb nitro glycerine or such other nitric esters or plasticizers during storage of the solid propellant grain or case bonded rocket motor, and eventually become highly combustible themselves, whereby inhibition becomes ineffective. In addition, the absorbed nitro glycerine, nitric esters or plasticizers cause swelling, softening and loss of rigidity whereby, during combustion of the propellant concerned, the overall mechanical rigidity of the charge, depending as it does to a considerable extent on the rigidity of the inhibitor, decreases progressively to such an extent that charge deformation occurs causing contraction of the conduit area. The result is usually failure of the motor, arising from the increased pressure which is consequent upon the conduit cross-sectional area becoming less than the nozzle throat area and so effectively acting as a choke. Further such swelling and softening of the cellulosic inhibitor material renders it prone to erosion by hot gas flow within the rocket motor chamber, leading to progressive exposure of propellant surface in an unpredictable, uncontrolled fashion with consequent uncontrolled rises in motor pressure.
If cellulose derivatives are replaced by elastomeric material and this is used as the inhibiting material, during combustion inhibiting material which is no longer supported by propellant will readily be displaced from its original configuration and will then exert peeling forces on inhibiting material still bonded to and inhibiting the combustion of propellant surfaces. The peeling forces frequently result in the pulling away of large areas of inhibiting material from the surfaces of unburnt propellant, whereby these surfaces become free to combust. Failure will usually result.
Thus it is an object of the invention to provide an improved method of inhibiting solid propellant surfaces.